1. Fieid of the Invention
The present invention relates to a diamond film coated cutting tool and a manufacturing method thereof, and in particular to a diamond film coated cutting tool which provides and excellent adhesive force enabling machining of a hard workpiece.
2. Description of the Background Art
Generally, a workpiece which is not easily machined by conventional tools is a nonferrous metal such as Alxe2x80x94Si alloy or Cu, a composite material, and a semi-sintered material such as a graphite, a polymer composite material, ceramics, etc. In order to machine these kinds of hard workpieces, a diamond which has the highest hardness value among the existing materials is generally used. Heretofore, the polycrystalline diamond (PCD) cutting tools, which are obtained by brazing a blank (a sintered diamond) to a cemented carbide insert, are widely used for machining certain materials such as Alxe2x80x94Si alloy, etc, which are used as vehicle engine materials. However, a PCD is expensive, and the shape of the same is simple; therefore, it can only be used as an insert shaped tool. In addition, it is very difficult to manufacture a chip breaker which is used for adjusting the chip shape of a workpiece during a cutting operation, and it is difficult to manufacture a tool of complicated shapes like an end mill, drill, reamer, etc., so that the polycrystalline diamond cutting tool is not applicable widely.
In order to completely overcome the above-described problems of the polycrystalline diamond cutting tools, a method for vapor-depositing a diamond film on a substrate was disclosed. Namely, when depositing a diamond film on a certain shaped substrate (insert of a chip breaker type, grill, end mill, etc.) so that the deposited diamond film may have a diamond structure, it is possible to obtain a superior wear-resistance to the conventional polycrystalline diamond tool and to decrease the fabrication cost significantly.
A cemented carbide material, consisted of a tungsten carbide(WC) grain and a cobalt(Co) which is a binder material, is an ideal substrate material for a coated tool. As a binder, Co is added by 6xcx9c20 wt %, and other carbide materials such as TiC, TaC, NaC, VC, etc. are added by a few wt % to tens wt % in order to control a mechanical properties such as a toughness, a Near-resistance, etc. Since this material has a toughness adapted for a tool, it is generally used as a substrate material for a coated tool. Thus, it is very important to develop a cutting tool which is coated with diamond on the cemented carbide material having the above-mentioned properties and a fabrication method thereof.
The kinds of chemical vapor deposition (CVD) methods for coating the diamond film are a hot filament CVD, a microwave plasma CVD, a DC plasma CVD, a DC arc-jet CVD, etc, and the diamond film is coated in a state of dissociating a mixed gas of a hydrocarbon such as methane, etc and hydrogen by plasma or thermal energy. When depositing the diamond film on the cemented carbide substrate using the above-described method, the biggest problem is that an adhesive force between the diamond film and the substrate material is very weak irrespective of the deposition method of the diamond film. Subsequently, a premature flaking of the diamond film, i.e. the diamond film having a weak adhesive force is delaminated before a full wear of the diamond film is made, occurs. Therefore, a method to prevent delamination problems of the diamond film during the use of the tool must be developed priorly by enhancing the adhesive force between the diamond film and the cemented carbide substrate material.
In order to overcome the above-described problems, various methods were disclosed for enhancing the adhesive force between the diamond film and cemented carbide substrate material. These methods are based on eliminating any effects of the Co used as a binder material in the cemented carbide substrate material, and enhancing a surface roughness of the substrate material to increase the mechanical interlocking with the diamond film. Since Co acts as a catalyst in transforming a diamond to graphite, Co expedites the graphitization of the diamond during a deposition of the diamond, thereby decreasing the adhesive strength of the diamond film. Therefore, it is required to eliminate any effects of the Co for a diamond film coated tool.
Considering the above-described points, there is provided a method of etching the Co phase of a cemented carbide surface portion by a certain depth to eliminate any effects of the Co, and then etching the cemented carbide grain using a Murakami solution to protrude the cemented carbide grain from the surface, so as to enhance the mechanical interlocking between the diamond film and the substrate material. That is, the etching of the cemented carbide grains as well as the etching of the Co phase makes the surface of the substrate material protruded, and forms many fine windings so as to enhance the mechanical interlocking between the diamond film and the substrate material. The adhesive force of the diamond film can be increased from 60 kg to 100 kg as confirmed by the a Rockwell A indentation test. The etching thickness of the Co layer by 2xcx9c15 xcexcm is required so as to completely eliminate any effects of the Co when the diamond film is coated. Therefore, the portion in which the Co phase acting as the binder material exists remains in a three-dimensionally connected holes (similarity to void in a sponge) by the etching depth from the surface. These voids may decrease the mechanical strength of the surface of the substrate material for thereby causing premature flaking of the diamond, therefore decreasing the life span of the tool. When checking the performance test of a commercial diamond film coated cutting tool, the cutting performance has a wide distribution. In particular, in the case of an interrupted cutting operation or cutting operation of Al alloy having a large amount of Si, the premature flaking of the diamond film is too high.
Also, in order to enhance a mechanical interlocking with the diamond film, there are provided a method of heat treating the substrate under a vacuum or an inert gas atmosphere to significantly decrease the concentration of Co phase on the substrate surface and a method of carburizing cemented carbide grains of the substrate surface to refine the cemented carbide grains or grow a cemented carbide grain for thereby increase the cemented carbide grain size. U.S. Pat. No. 5,623,256 discloses a method of easily evaporating Co of the cemented carbide grains surface by a heat treatment under a nitrogen or hydrogen atmosphere, and a method of forming Co carbides using a carbon atmosphere in a vacuum furnace and then passivating the carbides. In addition, the U.S. patent discloses that during the heat treatment, the grains are grown, which is used for increasing the adhesive force of the diamond film. U.S. Pat. No. 5,068,148 discloses that when the cemented carbide substrate is polished into a final shape, the cemented carbide grains of the substrate surface exist in a broken shape, and that since the diamond film is coated thereon thereafter, the adhesive force of the diamond film is decreased. Therefore, the U.S. patent provides a method of heat-treating the substrate materials at 1000xc2x0 C. to 1600xc2x0 C. under a vacuum or a non-oxidizaing atmosphere to melt the broken cemented grains for thereby forming the grown cemented carbide grains on the substrate surface. Thus, the adhesive force of the diamond film increases. However, in order to eliminate any effects of the Co, this patent employs an etching as well as heat treatment method to elimintate the Co layer of the substrate surface.
In a similar manner as U.S. Pat. Nos. 5,623,256 and 5,068,148, U.S. Pat. No. 5,585,176 discloses a method of heat-treating the cemented carbide substrate material at 1510xc2x0 C. under a nitrogen atmosphere to increase the cemented carbide grain size of the substrate surface and increase the surface roughness by 25xcx9c40 pinches, and then coating the diamond film, so as to obtain a diamond film coated tool having a thickness of 22xcx9c100 xcexcm with an excellent adhesive force. In this method, the adhesive force of the diamond film amounts to 100 kg by the Rockwell A indentation test without premature flaking. In addition, in this method, since voids are not formed at the portion in which the Co binder phase exists unlikely from the etching method, it is possible to prevent the decrease of mechanical strength of the substrate material in the grain boundary area. However, this patent has a problem that the substrate grain size of the internal portion as well as the substrate grain size of the surface portion is increased so that the grain size is distributed 1 to 11 xcexcm, and that the heat treatment time longer than 2xcx9c3 hours is required. That is, the substrate grain growth of the internal portion is unavoidable. When the substrate grain size of the internal portion is changed during the heat treatment, the mechanical properties of the substrate material may also change. Therefore, in order that a fine structure should not be changed so as not to change the mechanical proprerties of the substrate material, it is required to grow only the desirable grains within a short period at a temperature as low as possible.
Also, this patent has a limit in the chemical composition of an adaptable substrate material. First, other carbide materials such as Ti, Ha, Ta, Nb, V, Mo, Cr must be added to WCxe2x80x94Co by below 1 wt %, because in the case that an additive is contained more than the specific amount, it is difficult to obtain the grain growth on the surface. In addition, the content of Co is limited to preferably no more than 7%.
The problems for enhancing the adhesive force of the diamond film coated cemented carbide tool is as follows. First, there is a limit in the cemented carbide substrate material for coating. As the utilization condition of the tool such as an interrupted cutting operation becomes difficult, a tool with a good toughness is required. Therefore, the cemented carbide substrate material with a composition of 10% to 15% Co is required. However, the present technologies applies only to C2 grade in which the Co content is no more than 7%. Second, as described above, the contents of other carbide added to the cemented carbide is limited.
Consequently, the effectiveness of etching method and heat treatment method for adhesion enhancement depends on the composition of other carbide. Therefore, even in the same C2 grade, the type of the substrate materials is limited. Third, the present technologies depends largely upon the cemented carbide grain size of the substrate material. Therefore, the present technologies applies only to the substrate material having a size of the cemented carbide grain no less than 1 xcexcm. However, a method applicable to a micrograin cemented carbide substrate material composed of a sub-micron size cemented carbide material is required.
That is, the method for enhancing the adhesive force between the diamond film and the cemented carbide substrate is intensively studied by many researchers. However, a manufacturing method for the diamond film coated tool with an excellent adhesive force which is capable of providing an excellent toughness without any limit in the amount of other carbides in the substrate material and in the cemented carbide grain size of the substrate material, and having little problems for a cutting operation has not yet been developed.
Accordingly, it is an object of the present invention to provide a diamond film coated cutting tool and a manufacturing method of the same which is capable of providing an excellent toughness and a sufficient adhesive force without any limit in the amount of other carbides in the substrate material and Co and in the size of the cemented carbides grains.
In order to achieve the above object, there is provided a diamond film coated cutting tool, comprising a surface layer which cemented carbide grains are grown abnormally on the cemented carbide substrate, and a diamond film formed on the surface layer.
There is also provided a method for manufacturing a diamond film coated cutting tool, comprising the steps of heat-treating a surface of a cemented carbide substrate under a decarburizing atmosphere until the surface changes to a xcex7 phase, heat-treating the surface-decarburized cemented carbide substrate under a carburizing atmosphere to grow surface grains of cemented carbide abnormally, and depositing a diamond film on the carburized surface of the cemented carbide substrate.